1. Field of the Invention
The present invention relates to an optical disc device for recording a signal in an optical disc and/or reproducing a signal from an optical disc, a method for activating the optical disc device, and a control circuit.
2. Description of the Background Art
As an approach for increasing the recording density of an optical disc, there has been proposed an optical disc device incorporated with an optical head, wherein a condensing system having a large numerical aperture is constructed by combining an objective lens and a solid immersion lens (hereinafter, called as SIL).
According to a system (hereinafter, called as SIL system) using an SIL, a material having a large refractive index (from about 1.8 to 2.0) is used for an SIL and a protective layer of an optical disc, and a recording operation and a reproducing operation are performed, using emission light from the SIL, which is obtained by performing gap control of setting the gap between the SIL and the protective layer of the optical disc to such a small value as about 25 nm.
The numerical aperture of the condensing system is about 1.8, which is about twice of the numerical aperture of BD (Blu-ray Disc).
Using such a large numerical aperture may reduce a dynamic range capable of detecting a focus error signal required in focus control of setting a condensing position (hereinafter, also called as “focus position”) of emission light from the SIL on a targeted information recording/reproducing surface of an optical disc, in other words, reduce an allowable range of the focus position with respect to a targeted information recording/reproducing surface.
According to a result of recent research conducted by the inventors of the present application, the dynamic range in the SIL system is extremely narrow i.e. a range of about ±0.12 μm to ±0.24 μm with respect to a targeted information recording/reproducing surface as a reference surface.
It is necessary to set the focus position in advance within a dynamic range capable of detecting a focus error signal to perform focus control with respect to a targeted information recording/reproducing surface. However, as described above, the dynamic range in the SIL system is extremely narrow. Accordingly, considering mounting precision of an SIL and the other optical element constituting an optical head, thickness precision of a protective layer of an optical disc, and a like factor, it is hardly likely that the focus position may be constantly set within the dynamic range capable of detecting a focus error signal with respect to a targeted information recording/reproducing surface.
In view of the above, it is essentially important to provide a focus position adjusting mechanism for setting the focus position within the dynamic range in an optical head.
Employing means similar to a spherical aberration correcting mechanism for BD is effective as a measure for realizing the focus position adjusting mechanism, as recited in e.g. Japanese Unexamined Patent Publication No. 2002-157750.
Specifically, in the above publication, a beam expander constituted of two lenses i.e. a concave lens and a convex lens is provided in the optical head, and one of the lenses is moved in the optical axis direction. Thereby, a change in the divergent degree of beam is converted into a change in the condensing position of beam transmitted through the SIL.
In the following, a focus position adjusting mechanism using a beam expander constituted of two lenses i.e. a concave lens and a convex lens in the SIL system is described referring to FIGS. 7 and 8. In FIG. 8, a feed mechanism constituted of a feed stage 15, a feed screw 16, and a stepping motor 17 is not illustrated.
Referring to FIG. 7, the reference numeral 1 indicates a light beam, and 2 indicates an optical axis of the light beam 1. 3 indicates a concave lens, and 4 indicates a convex lens. The concave lens 3 and the convex lens 4 constitute a beam expander. 5 indicates an objective lens, and 6 indicates an SIL. The objective lens 5 and the SIL 6 constitute a condensing system. 7 indicates an optical disc, 8 indicates one of the information recording/reproducing surfaces of the optical disc 7, 13 indicates an upper surface of the optical disc 7, and 14 indicates a protective layer of the optical disc 7. The protective layer 14 is formed in a region from the information recording/reproducing surface 8 to the upper surface 13.
In the above arrangement, the concave lens 3, the convex lens 4, the objective lens 5, and the SIL 6 are arranged at such positions that the respective centers thereof are aligned with the optical axis 2. The reference numeral 9 indicates a distance (hereinafter, called as a gap) between the upper surface 13 of the optical disc 7 and the SIL 6. The objective lens 5 and the SIL 6 are connected to each other by a fixing member 10. Controlling the objective lens 5 and the SIL 6 by driving an actuator 11 enables to perform gap control of setting the gap 9 to a constant value.
In FIG. 7, solely the gap control by controlling the actuator 11 i.e. position control in vertical direction with respect to the optical disc 7 is described. Tracking control is enabled by providing an actuator other than the actuator 11, and performing position control in horizontal direction with respect to the optical disc 7. The reference numeral 12 indicates an actuator for driving the concave lens 3 in the direction of the optical axis 2. The actuator 12 is operable to perform focus control in response to supply of a predetermined current.
The reference numeral 15 indicates a feed stage. The feed stage 15 is connected to the stepping motor 17 via the feed screw 16, and is operable to integrally move the concave lens 3 and the actuator 12. In this arrangement, in response to supply of a predetermined drive pulse to the stepping motor 17, the feed stage 15 is moved in the direction of the optical axis 2. Thereby, the concave lens 3 and the actuator 12 are integrally moved in the optical axis direction.
Referring to FIG. 7, assuming that a targeted information recording/reproducing surface is the information recording/reproducing surface 8, the focus position is not set on the information recording/reproducing surface 8 or its vicinity. In this state, it is judged that the focus position is not set within the dynamic range capable of detecting a focus error signal with respect to the information recording/reproducing surface 8.
FIG. 8 is a diagram showing a state that the concave lens 3 is moved along the direction of the optical axis 2 by a predetermined distance rightwardly on the plane of FIG. 8, as compared with the state shown in FIG. 7, by operating the feed mechanism constituted of the feed stage 15, the feed screw 16, and the stepping motor 17.
As is obvious from FIG. 8, the divergent degree of a light beam on the right side of the convex lens 4 on the plane of FIG. 8 is smaller than that in FIG. 7, because the distance between the concave lens 3 and the convex lens 4 is decreased. Thereby, the condensing position i.e. the focus position of a light beam through the objective lens 5 and the SIL 6 is shifted to the left position on the plane of FIG. 8, as compared with the state shown in FIG. 7. In this state, it is judged that the focus position is set near the targeted information recording/reproducing surface 8 i.e. within the dynamic range capable of detecting a focus error signal with respect to the information recording/reproducing surface 8.
Although not illustrated, a focus error signal is detected by synthesizing output signals from a photoelectrical converter for detecting the focus error signal, using certain means e.g. an astigmatism method, and a control current obtained by subjecting the focus error signal to a predetermined electrical processing is supplied to the actuator 12. Thereby, focus control with respect to the information recording/reproducing surface 8 is established, and the focus position can be set on the information recording/reproducing surface 8 while following a thickness error of the information recording/reproducing surface 8.
Accordingly, providing the focus position adjusting mechanism as described above referring to FIGS. 7 and 8 in an optical head constituting an optical disc device meets the requirements on focus control for performing a desirable recording/reproducing operation with respect to a targeted information recording/reproducing surface.
However, the following drawback may occur in the conventional focus position adjusting mechanism as described above referring to FIGS. 7 and 8.
Specifically, as shown in FIG. 9, in the case where the concave lens 3 is moved in the direction of the optical axis 2 together with the actuator 12 by the feed mechanism constituted of the feed stage 15, the feed screw 16, and the stepping motor 17 for focus position adjustment, the moving direction of the concave lens 3 may not be in perfect parallel alignment with the optical axis due to a failure in rectilinear movement of the feed mechanism.
FIG. 9 is a diagram showing the above state, wherein the concave lens 3 is displaced to e.g. an upper right position on the plane of FIG. 9, and displacement (decentering) occurs between the center 20 of the concave lens 3 and the optical axis 2. As a result, whereas the light beam 1 is vertically symmetrically with respect to the optical axis 2 on the plane of FIGS. 7 and 8, the light beam 1 transmitted through the concave lens 3 is vertically asymmetrically with respect to the optical axis 2 on the plane of FIG. 9.
If a light beam whose symmetry with respect to the optical axis 2 is lost is transmitted through the condensing system constituted of the objective lens 5 and the SIL 6, coma aberration may occur. As a result, the light beam 1 may be condensed on the targeted information recording/reproducing surface 8 of the optical disc 7 with a certain divergent degree, without being converged into a single point, as shown in FIGS. 7 and 8.
It is well known that generation of coma aberration may result in a seriously adverse effect on recording/reproducing characteristics of an optical disc. Therefore, it is required to eliminate coma aberration in an optical disc device.
The adverse effect by coma aberration is deterioration of follow-up performance with respect to a decentered track, or an increase in the jitter of a reproduction RF signal resulting from distortion in the shape of a beam spot, because a tracking error signal of a sufficiently large amplitude is not obtained, and a gain in tracking control is reduced.
Coma aberration in a general optical disc device for BD can be corrected by changing a relative tilt between an optical disc and an objective lens. However, in the SIL system, as described above, the gap between the SIL 6 and the optical disc 7 is as small as 25 nm. Accordingly, employing a method corresponding to coma aberration correction to be used in a general optical disc device for an optical disc, i.e. a method comprising changing a relative tilt between the SIL 6 and the optical disc 7 is not desirable, because there is a likelihood that the SIL 6 may collide against the optical disc 7.
In focus position adjustment in the SIL system, as far as the moving distance of the concave lens 3 is so small as not to generate coma aberration, the aforementioned drawback is negligible. However, as a result of research conducted by the inventors of the present application, if the focus position adjustment is performed in a range of 20 μm, a moving distance required for the concave lens 3 is as large as about 2 mm. Accordingly, a decentered amount corresponding to the moving distance is about 40 μm, considering mechanical assembly precision or a like factor. If coma aberration by decentering is converted into wavefront aberration, the wavefront aberration is as large as 150 mλ, which greatly exceeds 20 mλ as an allowable value of wavefront aberration for performing a normal recording/reproducing operation.
The value of 20 μm corresponds to a distance from an information recording/reproducing surface as an uppermost layer to an information recording/reproducing surface as a lowermost layer of an optical disc having a multi-layered structure, as a result of research conducted by the inventors of the present application.
If a non-spherical aberration system is constructed by using an aspherical lens as the concave lens 3 and the convex lens 4, a slight decentering may cause a large coma aberration. As a result, it is extremely difficult to realize an optical disc device having a desirable arrangement.